Existing conventional centralized power systems generally include a single utility that supplies power to a grid, where users (commercial and residential power consumers) are connected to the grid. Electrical service can thus be provided by the utility to the consumers. Such centralized systems are beneficial in that they do not require a comprehensive power management scheme. This is because there is only one utility providing power.
However, power shortages and deregulation of electric companies in the United States has given rise to increased demand for independent energy sources that can co-exist with the electric utility, and supplement its capacity. Independent energy sources include, for instance, photovoltaics, wind, hydro, fuel cells, storage systems (e.g., batteries), superconductors, flywheels, capacitors, diesel engines, Stirling engines, gas turbines, and micro-turbines.
Given the availability of such independent energy sources, a typical goal in energy production is to interconnect any number of independent energy sources together with the central utility power source on a single power grid, or to develop smaller local area grids with multiple independent energy sources. In such a decentralized, distributed power generation system, each of the independent energy sources draws power from the grid when needed, and delivers power to the grid when an excess exists. This arrangement effectively supplements the capacity of the central utility, and enables the independent sources to buy and sell their energy.
However, unlike a centralized system, a decentralized local grid power system requires interconnection of the multiple energy sources and a comprehensive management scheme. For example, the proper grid voltage must be maintained with multiple power sources connected to the grid. In addition, large scale integration of independent power sources requires an interface between the grid and the power sources to regulate the power from/to the grid, and to monitor and regulate the activity. Each independent source's power must be converted to have parameters (e.g., phase and magnitude) that accommodates the grid parameters.
In addition to power conversion requirements, there is also a requirement that each independent source must be adapted to disconnect from the grid when the grid becomes unstable in order to avoid islanding. Islanding, also referred to as run-on, is the continued operation of a grid-coupled power converter, generator, or independent power source when the utility grid has been switched off, cut-off, or the distribution lines have been damaged.
Under such conditions, no power is delivered from the utility side and all independent sources should be disconnected from the grid. Such disconnects are intended to prevent the power of an independent source from feeding back through the grid to other power lines. The independent source can still supply power to its load if necessary. Without such disconnects, unsafe conditions may result, such as hot power line that is supposed to be de-energized for repairs.
Power quality is also a concern, as the voltage, frequency, harmonic content, and other parameters of the grid might go outside acceptable bounds. This condition presents a hazard to loads on the grid, such as computers and other sensitive electronics that might be damaged by an unstable or otherwise non-conforming grid. Moreover, the distributed generating equipment connected to the grid can be damaged due to grid parameter fluctuations, which may further present a safety hazard to the property and persons nearby such equipment.
Conventional schemes that have been proposed or implemented to eliminate the islanding problem, however, have only achieved marginal success and are generally considered unreliable. In particular, each of the available techniques are associated with non-detect zones, where islanding may still occur given a particular set of conditions. Thus, even though such schemes may work some percentage of the time, the associated risks to life and property remain substantial.
Many of the potential grid faults that could interrupt the flow of power from utility power stations to consumers could be repaired safely without affecting consumers if there was a reliable anti-islanding technique. It would allow the local reticulation lines, or any grid connected inverters connected to them, to be powered down safely with no risk of electrical shock.
What is needed, therefore, is an anti-islanding technique that reliably eliminates islanding conditions in all likely distributed generation situations.